In extension of our previous study on particle size dependent solubility limits in nano-sized LiFePO4 we wish to extend our study to temperature dependent solubility limits in nano-sized Li(Fe/Mn)PO4. We hope to further verify our recently developed thermodynamic models for nano-insertion materials and provide more insight in the thermodynamics of these materials. (1) We wish to determine the solubility limits as a function of temperature, and to determine the solubility limits as a function of overall starting composition for nano-sized LiFePO4 materials, and test the validity of the debated Vegard¿s law in LiFePO4. (2) Secondly we would like to determine if our nano-sized LixMnPO4 (with a crystallite size down to 40nm) has stable solid solution compositions at elevated temperature especially in the light of promising electrochemical behaviour recently reported for this material.